1. Field of the Invention
This invention relates to combustion systems employing burners that produce highly luminous flames, thereby providing higher heat transfer and lower NOx emissions than conventional combustion systems. More particularly, this invention relates to a method and apparatus for producing substantially flat flames which produce uniform heat distribution and relatively high radiative heat transmission. Burners employed in the combustion systems of this invention preferably use oxygen or oxygen-enriched air as an oxidant, although air may also be used.
2. Description of Related Art
Environmental requirements have been and continue to be a major impetus for developing new combustion methods and apparatuses. Manufacturers are being forced to reduce emissions, all the while striving to control costs and maintain product quality. For example, industry is faced with the need to reduce NOx emissions significantly. In response thereto, advanced combustion systems, oxygen-enrichment and oxygen-fuel combustion, are being developed. By way of example, U.S. Pat. No. 5,725,366 describes a method and apparatus for combustion of a fuel/oxidant mixture in which at least a portion of the fuel is preheated and, thereafter, burned with any remaining portion of fuel in a flame having fuel-rich zones, thereby forming soot within the resulting flame to produce a luminous, high heat transfer, low-NOx flame. See also U.S. Pat. No. 4,909,727 which describes a combustion process in which a portion of the fuel to be burned is first cracked using oxygen-enriched air to produce a cracked fuel, which includes a soot component, which is subsequently introduced into a combustion chamber with a second portion of fuel to produce a highly luminous flame.
Combustion technology involving the use of fuel-oxygen systems is relatively new in glass melting applications. Conventional burners typically employ a cylindrical burner geometry in which fuel and oxygen are discharged from a cylindrical nozzle, such as a cylindrical refractory block. Such cylindrical discharge nozzles produce a flame profile that diverges in a generally conical shape. However, conventional burners that produce generally conical flames have the undesirable tendency to produce hot spots within the furnace, resulting in furnace refractory damage, particularly to furnace crowns or roofs and sidewalls which are opposite the flame. Such conventional burners also cause increased raw material volatilization and uncontrolled emissions of nitrogen oxides, sulfur oxides and process particulates.
To address some of the problems associated with such designs, conventional burners have incorporated low momentum flow, which is produced by the use of relatively lower fuel and oxygen velocities, resulting in relatively lower momentum flames. Such lower velocities and, thus, lower momentums result in longer flames and increased load coverage. However, undesirable flame lofting occurs at such lower velocities, causing undesirable effects.
Some conventional combustion systems employ a staggered firing arrangement in an attempt to improve effective load coverage, particularly with the use of conical expansion of individual flames. However, this staggered firing arrangement often creates undesirable cold regions in pocket areas between adjacent burners. Increasing the number of burners employed, thereby increasing flame coverage, has been employed as a means for addressing this issue. However, increasing the number of burners also undesirably significantly increases the installation and operation costs. These issues are addressed, for example, by the method and apparatus of U.S. Pat. No. 5,545,031 in which a fishtail or fan-shaped flame which produces uniform heat distribution and relatively high radiative heat transmission is employed. The fuel is discharged from a nozzle in a generally planar fuel layer, forming a fishtail or fan-shaped fuel layer having generally planar upper and lower boundaries. Oxidant is discharged from the nozzle so that a generally planar oxidant layer is formed at least along the upper boundary of the fuel layer and preferably also along the lower boundary of the fuel layer.
Notwithstanding the improvements that have been made to date, there still remains a need for a burner system that can be employed in high temperature furnaces, such as glass melting furnaces, that provides uniform heat distribution, reduced undesirable emissions, such as nitrogen oxides and sulfur oxides, and which produces highly radiative and luminous flames. In addition, industrial burner operators continue to desire reliability, simplicity and low cost in the equipment employed. Typically, this means that a combustion system, in order to be accepted, must be a retrofit design, there must be only one oxygen and one fuel supply to the burner, and fuel and oxygen mixing within the preheating zone of the burner must be rapid to prevent the burner from becoming too physically large.